Field of the Invention
The present invention relates to electric rotary shavers and in particular, to a cutter unit for such shavers having an improved configuration. The invention also relates to a method of manufacturing a cutter and cap for such a cutter unit.
Description of the Related Art
In electric rotary shavers, a cutter unit is formed by an outer cutting element which interacts with a rotating inner cutting element or cutter to trap and cut hairs. The outer cutting element, often known as the cap is generally in the form of an annular shaving track having slots through which the hairs may protrude. The inner cutting element or cutter is generally shaped as a disk having a plurality of upstanding legs carrying blades. The cutter rotates whereby the blades follow the annular track and interact with the slots to cut the hairs. A rotary shaver may have one, two or more such cutter units carried in a shaving head. A popular design is the three cutter head in which three such cutter units are arranged in a triangular configuration. A device of this type is shown in U.S. Pat. No. 5,408,749.
A key feature in the design of a shaving head is the comfort to the user. This requires that cutting takes place smoothly, irrespective of which part of the head is being used, the pressure applied and the direction of movement of the head across the skin. The cutter should cut the hairs cleanly without snagging and skin should not be drawn into the slots or otherwise come into contact with the rotating cutter surfaces. Important considerations in achieving comfort are the cutter or shearing angle enclosed between the front surface of the cutter and the inner surface of the cap. The cutter's legs and front surface are usually angled upwards at a cutter angle of typically 40° to 50° relative to the inner surface of the cap to decrease cutting forces. The cutting edge will typically lie in a radial direction while the slots in the cap may be at a shearing angle of typically 3° to 7° to the cutter. This helps to provide smooth running of the cutter surfaces over the lamellae of the shaving track because the rotating surfaces will always be in contact with the lamellae at some point. Considerable care is taken in achieving the optimal angles for any given cutting configuration.
It has recently been noted that toroidal, domed or donut shaped shaving tracks in the caps are comfortable to the skin. This means that the skin-contact surface of the shaving track is not flat or planar, but instead domed. A device disclosing a domed shaving track is described in CA2536424.
Although such configurations may improve comfort in some aspect they have led to further difficulties in relation to maintaining a desired shearing angle. In particular for a domed track, conventional angled cutters have a curved cutting edge and are unable to maintain a constant shearing angle across the width of the shaving track when the domed shaving track has straight hair-entry slots. This effect may partly be overcome by making the slots in the cap non-linear too, to match the cutter. Such adaptations are however costly, since cutting and grinding procedures required to produce curved slots in the cap material are far more complex than those which may be used to form straight slots.
It would therefore be desirable to provide a cutter unit having a configuration that would at least partially overcome some or all of the above inconveniences.